Brake pressure controller for a hydraulic vehicle brake system

ABSTRACT

A brake pressure controller for a hydraulic vehicle brake system includes a master brake cylinder (1), a wheel brake cylinder (10, 11) connected thereto through a brake conduit (5), and a branch conduit (8, 9) which is provided in the brake conduit (5) between the master brake cylinder (1) and the wheel brake cylinder (10, 11). The brake system includes control valves (12, 14) and a pump (16) returning pressure fluid from a return conduit (15) into the brake conduit (5). At least one valve (6) is provided between the master brake cylinder (1) and the branch conduit (8, 9), which valve, in a first mode of operation, connects the outlet (17) of the pump (16) to the master brake cylinder (1) and, in a second mode of operation, discontinues the connection from the outlet (17) of the pump (16) to the master brake cylinder (1) and connects the inlet (24) of the pump (16) to the master brake cylinder (1) or a reservoir (4). The inlet (24) of the pump (16) includes a pressure holding valve (20), the opening pressure of which, in the first mode of operation, is above atmospheric pressure and, in the second mode of operation, is reduced to permit an automatic intake of pressure fluid to the pump (16).

This application is a division of application Ser. No. 08/211,741, filed Aug. 25, 1994, now U.S. Pat. No. 5,486,040.

BACKGROUND OF THE INVENTION

The present invention is concerned with a brake pressure controller for a hydraulic vehicle brake system. The brake system includes a master cylinder, at least one wheel brake cylinder for actuating a brake and a brake conduit for providing pressure fluid communication between the wheel brake cylinder and the master brake cylinder. A brake pressure control circuit is provided in the brake conduit between the master brake cylinder and the wheel brake cylinder.

The brake system also includes a pump and control valves, controllable through a controller, which connect the wheel brake cylinder to the brake conduit or a return conduit leading to the inlet of the pump. The control valves include at least one valve provided in the brake conduit between the master brake cylinder and the brake pressure control circuit, which at least one valve, in a first operating mode, connects the outlet of the pump, through the brake conduit, to the master brake cylinder, and, in a second operating mode, discontinues the connection between the outlet of the pump and the master brake cylinder and permits a connection between the inlet of the pump and the master brake cylinder or a reservoir.

A brake pressure controller of the above-described type is known from German Patent No. DE-OS 38 31 426. The automatic control system of the brake pressure acting on the wheel brake cylinders responds to the rotating movement of one or several wheels of an automotive vehicle monitored by a monitoring system. The control system prevents the wheels from locking during a braking operation through brake slip control, and prevents the driven wheels from racing, during start-up, through traction slip control.

The first mode of operation is provided for brake slip control while the second mode of operation is provided for traction slip control. In known brake pressure control systems, the brake pressure cylinder, in the first mode of operation during a pressure reducing step for lowering the brake pressure, is connected only to a low-pressure accumulator and to the inlet of the pump. If the low pressure accumulator is empty, there is a danger that the pressure in the wheel brake cylinder may be lowered, through the intake force of the pump, to such an extent that a retraction of the brake piston and an ingress of air are likely to occur, which might affect the braking operation and the brake pressure control.

SUMMARY OF THE INVENTION

It is an object of the invention to improve a brake pressure controller of the above-mentioned type that, during operation, the pressure of a wheel brake cylinder cannot fall below atmospheric.

This problem, in the practice of the invention, is solved in that the inlet of the pump includes a pressure holding valve which is closed in the resting position. In the first mode of operation, the pressure holding valve requires, for opening, an excess pressure in the return conduit which is above atmospheric pressure. In the second mode of operation, the pressure holding valve requires only a slight opening pressure in the return conduit to enable an automatic intake of the pump.

The configuration according to the present invention prevents the brake pressure in the wheel brake cylinder from dropping below the opening pressure of the pressure holding valve which is above atmospheric pressure, during those instances of first mode of operation when the wheel brake cylinder is separated from the master brake cylinder. Even with a complete pressure decrease through corresponding control commands of the controller, a residual pressure of above 1 bar will be maintained in the return conduit so that an ingress of air or a withdrawal of the brake piston is effectively avoided.

In the second mode of operation, the wheel brakes of the driving wheels of the vehicle are controlled by the controller without actuating the master brake cylinder and, in this way, are automatically actuated. The required brake pressure is generated by the pump. The higher opening pressure of the pressure holding valve, which is operative in the first mode of operation, would prevent the automatic intake of pressure fluid from the master brake cylinder through the pump. However, in the invention, the automatic intake of pressure fluid through the pump is enabled in the second mode of operation. This is accomplished by reducing the opening pressure of the pressure holding valve in the second mode of operation of the brake pressure controller.

In one embodiment of the present invention, the pressure holding valve includes a spring-biased closure element to which is applied a first spring having a low opening force in the second mode of operation, and to which is applied a second spring having a higher opening force in the first mode of operation. A piston, responsive to a control pressure, in particular, the pressure of the master brake cylinder, aids in applying the second spring to the closure element. Thus, the pressure holding valve, depending on the mode of operation, is switched over to the appropriate opening pressure.

In a preferred embodiment of the invention, the pressure holding valve is the suction valve of the pump, and the piston consists of a cylindrical sleeve disposed in the pressure chamber of the pump between the pump piston and the suction valve. The ends of the piston have identical diameters sealed against the pressure chamber. The central section of the sleeve has a larger diameter confining an annular chamber to which a control pressure can be applied. To enhance the closing force, either an additional spring may be provided which, through displacement of the piston, will engage the closure element, or otherwise the preload of a single spring can be increased by displacing the piston. Moreover, the piston may be disposed in the intake chamber, with the effect of a spring acting in the opening direction for reducing the opening pressure, additionally, being variable by actuation of the piston.

According to another embodiment of the invention, the opening pressure of the pressure holding valve is unchanged and, in place, a hydrodynamic charging pump is disposed ahead of the pressure holding valve. In the second mode of operation, the charging pump generates a pressure in the return conduit exceeding the opening pressure of the pressure holding valve. In this embodiment, the pressure holding valve can also be formed as the suction valve of the pump, which would require the force of the valve spring acting on the closure member to be increased. As the opening pressure of the pressure holding valve is comparatively low, a hydrodynamic pump of a very simple design my be provided which is just able to overcome the opening pressure of the pressure holding valve and to feed, as a charging pump, the piston pump with pressure fluid provided for the brake pressure generation.

The hydrodynamic charging pump, in this embodiment, can be connected, in parallel to the pump and in the return conduit and be actuated together with the pump. The inlet of the hydrodynamic charging pump can be in communication with the master brake cylinder or the reservoir and, in the first mode of operation, this communication can be blocked through a valve closing in response to pressure. This embodiment involves the advantage that the charging pump does not require a separate driving motor and can be integrated into the pump.

In this embodiment, as the charging pump is actuated, the pump inlet, in the first mode of operation, is blocked by a valve to eliminate the charging effect. The valve is actuable in an electromagnetic or hydraulic manner. An embodiment in which the valve, through the pressure of the master brake cylinder, is switched into its blocking position, as through actuation of the master brake cylinder is particularly advantageous.

According to another embodiment of the present invention, a brake pressure controller may be used for a dual-circuit brake system having two separate circuits, each of which includes a pump and a pressure holding valve. For reducing the structural efforts, the intake conduits of the two brake circuits may be jointly connected to the outlet of the hydrodynamic charging pump, with the intake conduits of the two brake circuits being separated from one another by spring-loaded check valves. The independence of the two brake circuits required for safety reasons will, therefore, be maintained despite having a common charging pump.

According to another embodiment of the present invention, in the second mode of operation, the automatic intake of pressure fluid to the pump is enabled. In this embodiment, the pressure holding valve is disposed in the intake conduit upstream of the suction valve of the pump, and that between the suction valve and the pressure holding valve, a connecting conduit branches from the intake conduit. This connecting conduit leads to the master brake cylinder or a reservoir and contains a blocking valve which, in the first mode of operation, is closed and in the second mode of operation is open. In this embodiment, the pressure holding valve cannot replace the suction valve of the pump; however, the additional structural efforts in respect of measures eliminating the effect of the pressure holding valve in the second operating phase, are reduced.

The invention will now be described in detail with reference to some embodiments of the invention as shown in the drawing, wherein

FIG. 1 schematically shows a brake circuit of a vehicle brake system according to the present invention having a brake pressure controller of the invention including a pressure holding valve with a variable opening pressure,

FIG. 2 schematically shows a form of embodiment of the brake system according to FIG. 1 having a modified valve system in the brake circuit,

FIG. 3 schematically shows another embodiment of a brake circuit of a vehicle brake system of the present invention having a brake pressure controller and a pressure holding valve disposed ahead of the suction valve of the pump, and

FIG. 4 schematically shows another embodiment of the present invention for two brake circuits of a vehicle brake system and having a charging pump associated with the pressure holding valve.

The hydraulic vehicle brake systems as shown in FIGS. 1 to 4 have in common a master brake cylinder 1 comprising a vacuum brake force booster 2 actuable by a brake pedal 3. Provided on the master brake cylinder 1 is a reservoir 4 containing a pressure fluid stock and connected, in the brake release position, to the working chambers of the master brake cylinder 1.

The brake circuits as shown respectively comprise a brake conduit 5 in communication with a working chamber of the master brake cylinder 1. An electromagnetically actuable valve means 6, in its resting position, forms an open passageway for the brake conduit 5 and, through energizing the actuating magnet thereof, is switched into a blocking position in which the brake conduit 5 is blocked toward the working chamber of the master brake cylinder. In the opposite flow direction, the brake conduit 5 remains open through a check valve 7 connected in parallel to the valve means 6. Connected to the brake conduit 5 are branch conduits 8, 9 each leading to a wheel brake cylinder 10 and 11, respectively. The branch conduits 8, 9 each contain an electromagnetically actuable inlet valve 12 which, in its resting position, is open and, through energizing its actuating magnet, can be switched into a blocking position. Connected to each branch conduit 8 and 9 and in parallel to each inlet valve 12 is a check valve 13 opening toward the master brake cylinder 1.

Moreover, the wheel brake cylinders 10, 11, through the branch conduits 8, 9 are connected to electromagnetically actuable outlet valves 14 which are blocked in their resting position and, through energizing the actuating magnet thereof, can be switched into an open position wherein they connect the wheel brake cylinders 10, 11 to a return conduit 15. The return conduit 15 leads to the inlet of a hydrostatic pump 16 the outlet of which, through a pressure valve 17, is connected to the section of the brake conduit 5 in communication with the branch conduits 8, 9. The return conduit 15 is connected to a low pressure accumulator 18 taking up excessive pressure fluid during opening of the outlet valves 14 until the excessive pressure fluid is fed back through the pump 16. The return conduit 15, moreover, is in communication with a connecting conduit 19 which, in the example of embodiment according to FIG. 1, is or can be connected to the reservoir 4 and, in the reminder of the embodiments, is or can be connected to the section of the brake conduit 5 between the master cylinder and valve 6.

In the examples of embodiment as shown in FIGS. 1 and 2, the suction valve of the pump 16 is in the form of a pressure holding valve 20, the opening pressure of which is variable depending on the mode of operation. A part of a housing 21 of pump 16 contains a pressure chamber 22 formed by a cylindrical bore confined by the axially movable pump piston 23. The pressure chamber 22, through an inlet bore 24 provided in the front side thereof, is connected to the return conduit 15 and, through a lateral outlet bore 25, is connected to the pressure valve 17. The inlet bore 24 is closed through a valve ball 26 provided within the pressure chamber and supported, through two-series arranged compression springs 27, 28, on a piston 29.

The piston 29 is in the form of a cylindrical sleeve, the ends of which have identical diameters and are sealed against the bore forming the pressure chamber 22. Provided between the sealed ends of the piston 29 is an expanded, cylindrical annular chamber 30 in which a larger-diameter central section 31 of the piston 29 is axially displaceable and is sealed against the cylindrical wall of the annular chamber 30.

The central section 31, in this manner, confines an annular control chamber 32 which, through a control conduit 33 is in communication with the brake conduit 5 at a point between the master cylinder 1 and the valve means 6. The piston 29, through a control spring 34, is held in the position as shown in the drawing once the master brake cylinder is in an inactive position, e.g. in the brake release position. Located between the compression springs 27, 28 is a spring retainer which is supported on a stop sleeve 35 connected to the piston 29, thereby maintaining the compression spring 28 preloaded which spring 28 is stronger compared to compression spring 27.

Disposed in the connecting conduit 19 is a pressure-controlled valve 36, the control conduit 37 of which is in communication with the control conduit 33. The valve 36 is open in the resting position thereof and, through applying pressure to the control conduit 37, can be switched into a blocking position in which the connecting conduit 19 is blocked.

FIG. 1 shows the brake circuit of the brake system in the resting position in which the brakes VR, VL associated with the wheel brake cylinders 10, 11 located on the driven front axle of the vehicle, are not actuated.

To initiate a braking operation, the master brake cylinder 1 is actuated through the brake pedal 3 and the brake force booster 2, and a pressure is generated in the brake conduit 5 and in the branch conduits 8, 9 as well as in the wheel brake cylinders 10 coupled thereto. In this way, pressure is applied to the brakes VR, VL. In this first mode of operation of the brake circuit, the valve means 6 is not actuated so that it remains in the open position as shown.

The pressure in the brake conduit 5, through the control conduit 33, 37 is fed into the control chamber 32 and to the valve 36, thereby switching the valve 36 into its blocking position and pushing piston 29 to the right against the force of the control spring 34. This causes the compression spring 28 to directly act on the valve ball 26 and increases the pressure in the inlet bore required for opening the pressure holding valve 20 to about 1.3 bar.

If, during the braking operation, a control of the brake slip on the brakes VR, VL, is now required to preclude a blocking of the wheels, the outlet valves 14 and the inlet valves 12, in a known manner, are alternately actuated by the controller. Also, the pump drive of the pump 16 is activated. In the pressure reducing phase, through the closed inlet valves 12, the wheel brake cylinders 10, 11 are separated from the master brake cylinder 1 and, through the opened outlet valves 14, are in communication with the return conduit 15. The pressure fluid escaping from the wheel brake cylinders 10, 11 into the return conduit 15, first, is taken up by the low pressure accumulator 18 and is fed by pump 16 back into the brake conduit 5. Because of the higher opening pressure of the pressure holding valve 20, the pressure on the wheel brake cylinders 10, 11 cannot decrease below atmospheric pressure so that an ingress of air and a withdrawal of the brake piston are reliably avoided.

In the second mode of operation, the brakes VR, VL are automatically actuated by the controller once the master brake cylinder 1 is inoperative, in order to control, in a start-up process, the traction slip on the driving wheels and to prevent the driving wheels from racing.

In this respect, the valve means 6 is actuated by the controller, thereby separating the master brake cylinder 1 from the branch conduits 8, 9. At the same time, the drive of pump 16 is activated. As the control conduits 33, 37, in that mode of operation, remain non-pressurized, the valve 36 remains in the open position and the piston 29 remains in the basic position caused by the control spring 34. In that position, only the weak compression spring 27 is operative on the valve ball 26, which corresponds to a differential pressure of about 0.2 bar between the inlet bore 24 and the pressure chamber 22. That low differential pressure enables an automatic intake of pressure fluid through the pump 16 via the connecting conduit 19 from the reservoir 4. Thus, a supply of pressure fluid is conducted through the pressure valve 17 and the branch conduits 8, 9 to the wheel brake cylinders 10, 11, thereby actuating the brakes VR, VL.

The extent of actuation of the brakes is controlled through alternately actuating the inlet valves 12 and the outlet valves 14 as it is in a brake slip control. In this mode of operation, the return conduit 15, through the connecting conduit 19, is in permanent communication with the reservoir 4 to preclude a decrease in the pressure in the wheel brake cylinders to below atmospheric pressure during the pressure reducing phase. Accordingly, also in this operating phase, an ingress of air or a withdrawal of the brake piston will not take place.

The embodiment according to FIG. 2 distinguishes from the one according to FIG. 1 substantially in that the valve means 6 and the valve 36 are replaced by an electromagnetically actuated valve means 38 disposed in the brake conduit 5 in lieu of the valve means 6. The valve means 38 comprises a 2-way/3-position valve which, in the resting position thereof, forms an open passageway for the brake conduit 5 and blocks the nozzle of the connecting conduit 19. In its actuating position caused by energizing the actuating magnet, valve means 38 connects the connecting conduit 19 to the section of the brake conduit 5 leading to the master brake cylinder 1, and blocks the section of the brake conduit 5 in communication with the branch conduits 8, 9. In that actuating position of the valve means 38 enabling the second mode of operation, the connecting conduit is not directly connected to the reservoir 4 but rather through the working chamber of the master cylinder 1, as opposed to the embodiment according to FIG. 1. However, this does not affect the operation as the master cylinder 1 is not actuated during the second mode of operation so the working chamber is in communication with the reservoir 4.

If, during the second mode of operation, i.e. during a control of the traction slip, a braking process is initiated through actuation of the master brake cylinder, the brake pressure provided through the master brake cylinder 1 can propagate through the check valve 7, into the branch conduits 8, 9. At the same time, the actuation of the master brake cylinder 1 is detected by pressure-sensors or way sensors and is transferred to the controller, thereby causing a switch to the first mode of operation, restoring the valve means 6, 38 to the resting position.

In the embodiment according to FIG. 3, the pump 16 comprises a suction valve 39, the opening pressure of which is in the range of 0.2 bar so that pressure fluid can be automatically taken in by the pump 16. Disposed upstream of the suction valve 39 is a pressure holding valve 40, the opening pressure of which, invariably, is at about 1.3 bar. The connecting conduit 19 contains, as in the embodiment according to FIG. 1, a pressure-controlled valve 36, with one end connected to the return conduit 15 between the suction valve 39 and the pressure holding valve 40, and the other end connected to the section of brake conduit 5 in communication with the master brake cylinder 1.

In the brake controller according to FIG. 3, a pressure is generated in the brake conduit 5 in the first mode of operation through actuation of the master brake cylinder 1, which switches the valve 36 into the blocking position. The pump 16 can only take in pressure fluid through the suction valve 39 and the pressure holding valve 40, with the higher opening pressure of the pressure holding valve 40 preventing the pressure on the wheel brake cylinders 10, 11 from decreasing below atmospheric pressure.

In the second mode of operation, in which the master brake cylinder 1 is not actuated, the valve 36 will remain in the open position while the valve means 6 is switched into the closing position. Now, the pump 16, through the suction valve 39, the connecting conduit 19 and the brake conduit 5 can take in pressure fluid from the master brake cylinder 1, feeding pressure fluid the same to the wheel brake cylinder 10, 11 for automatically actuating the brakes. A pressure reduction for reducing the brake force is effected during opening of the outlet valves 14 in either mode of operation through the pressure holding valve 40 so that, during operation, the pressure in the wheel brake cylinders 10, 11, is not completely decreased down to atmospheric pressure. Only after termination of the respective braking operation, through return of the valve means 6 and the inlet valves 12 into their open resting position, a complete pressure decrease in the wheel brake cylinders 10, 11 is attained through flow-back of the pressure fluid to the master brake cylinder 1.

FIGS. 1 to 3 show three different alternatives for connecting the connecting conduit 19 to the master brake cylinder 1 and/or the reservoir 4 to permit the intake of pressure fluid through the pump in the second phase of operation. All three alternatives are of equivalent effect and are, therefore, respectively exchangeable.

The example of embodiment of FIG. 4 shows both brake circuits of a vehicle brake system. The two brake circuits are of identical design and correspond to the examples of embodiment described above. Connected to each brake circuit is a wheel brake cylinder 10 for actuating a front wheel brake and a wheel brake cylinder 11 for actuating a rear wheel brake such that the brakes associated to a brake circuit are located on the vehicle wheels disposed in diagonal relationship.

For brake pressure control purposes, each brake circuit is connected to a separate hydrostatic pump 16 the suction valve of which is formed as a pressure holding valve 40 of a constant, higher opening pressure. The two pumps 16 are driven by a common drive 41. To permit, in the second mode of operation for controlling the traction slip, an intake of pressure fluid through pumps 16, a hydrodynamic charging pump 42 is provided which is equally actuated by drive 41. The intake side of the charging pump 42 is connected to the branch of the brake conduit 5 of a brake circuit leading to the master brake cylinder 1 through a connecting conduit 19 containing a valve 36 switching in response to pressure. From the pressure side of the charging pump 42, a charging pressure conduit 43, respectively through a spring-loaded check valve 44, leads to the two return conduits 15 of the two brake circuits. The check valves 44 insure that the two brake circuits remain hydraulically separate from one another.

During a braking operation caused by actuation of the master brake cylinder 1, the valve 36 will be closed so that, in a brake slip control action, the charging pump 42 jointly driven with the pumps 16, cannot take in pressure fluid through the connecting conduit 19. Accordingly, pressure fluid is supplied to the pumps 16 only as long as the pressure in the return conduits 15 is above the opening pressure of the pressure holding valves 40.

If the brakes VL, VR are to be automatically actuated for controlling traction slip, after actuating the valve means 6 and activating the pump drive 41, the charging pump 42, through the open valve 36 and the connecting conduit 19, can take in pressure fluid from the master brake cylinder 1. Pressure fluid can then be fed through the charging pressure conduit 43 and the check valves 44, into the return conduits 15 of the brake circuits. This causes the pressure to rise in the return conduits 15 to such an extent that the pressure holding valves 40 are opened and the pumps 16 are enabled to feed, in the required manner, pressure fluid to the brake circuits. The charging pump 42 is so designed that the volumetric delivery thereof, at the pressure required for opening the pressure holding valves 40, exceeds the combined volumetric delivery of the two pumps 16, thereby insuring that each pump 16 receives an adequate amount of pressure fluid.

It goes without saying that an assembly can, equally, be used in which each hydrostatic pump 16 comprises a charging pump 42 of its own taking the pressure fluid from the brake conduit associated thereto. In an assembly of that type, both brake circuits remain completely separate so that check valves 44 are not needed. If only one circuit is required for controlling the traction slip because the brakes of the two driven wheels are connected to the same brake circuit, it will be adequate for only such brake circuit to be provided with a charging pump 42 as the other brake circuit is operated only in the first mode of operation.

LIST OF REFERENCE NUMERALS

1 master brake cylinder

2 vacuum brake force booster

3 brake pedal

4 reservoir

5 brake conduit

6 valve means

7 check valve

8, 9 branch conduit

10, 11 wheel brake cylinder

12 inlet valve

13 check valve

14 outlet valve

15 return conduit

16 pump

17 pressure valve

18 low pressure accumulator

19 connecting conduit

20 pressure holding valve

21 housing

22 pressure chamber

23 pump piston

24 inlet bore

25 outlet bore

26 valve ball

27, 28 compression spring

29 piston

30 annular chamber

31 larger diameter central section

32 control chamber

33 control conduit

34 control spring

35 stop sleeve

36 valve

37 control conduit

38 valve means

39 suction valve

40 pressure holding valve

41 drive

42 charging pump

43 charging pressure conduit

44 check valve 

We claim:
 1. A brake system for controlling fluid pressure in a wheel brake during normal braking, brake slip control and traction slip control, said brake system comprising:a brake pedal; a pressure fluid reservoir containing a pressure fluid; a master brake cylinder in fluid communication with said pressure fluid reservoir and responsive to movement of said brake pedal for developing a pressure build up of pressure fluid in said master brake cylinder in response to actuation of said brake pedal; a first and a second brake circuit each having:(a) a wheel brake, (b) first control means extending between said master brake cylinder and said wheel brake for:(i) controlling pressure fluid flow from and to said master brake cylinder, (ii) permitting pressure fluid flow between said master brake cylinder and said wheel brake during normal braking as said brake pedal is actuated and without sensing a tendency of wheel lock, (iii) regulating pressure fluid flow to said wheel brake during brake slip control as a tendency of wheel lock is sensed, and (iv) preventing pressure fluid flow through said first control means to said master brake cylinder during traction slip control as wheel slippage is sensed, (c) a hydrostatic pump having an intake and an outlet, (d) a return conduit extending between said wheel brake and said intake of said hydrostatic pump, (e) a low pressure accumulator containing pressure fluid disposed in said return conduit, (f) an outlet valve disposed in said return conduit between said wheel brake and said low pressure accumulator for:(i) preventing pressure fluid flow from said wheel brake during normal braking as said brake pedal is actuated and without sensing a tendency of wheel lock, (ii) regulating pressure fluid flow from said wheel brake during brake slip control as a tendency of wheel lock is sensed, and (iii) regulating pressure fluid flow from said wheel brake during traction slip control as wheel slippage is sensed, (g) a pressure holding valve disposed in said return conduit between said intake of said hydrostatic pump and said low pressure accumulator for permitting pressure fluid flow from said wheel brake through said return conduit and to said hydrostatic pump only when the pressure of said wheel brake exceeds an opening pressure, and (h) third control means extending between said outlet of said hydrostatic pump and said first control means for conducting pressure fluid flow from said outlet of said hydrostatic pump to said first control means during brake slip control and traction slip control; a charging pump having an intake and an outlet for generating a pressure exceeding the opening pressure of said pressure holding valve; a connecting conduit extending between said intake of said charging pump and said master brake cylinder; a valve disposed in said connecting conduit for(a) preventing pressure fluid flow between said pressure fluid reservoir and said intake of said charging pump during normal braking and brake slip control, and (b) permitting pressure fluid flow between said pressure fluid reservoir and said intake of said charging pump during traction slip control; a charging pressure conduit extending between:(a) said outlet of said charging pump, (b) a first point in said return conduit of said first brake circuit between said low pressure accumulator of said first brake circuit and said pressure holding valve of said first brake circuit, and (c) a second point in said return conduit of said second brake circuit between said low pressure accumulator of said second brake circuit and said pressure holding valve of said second brake circuit,said charging pressure conduit for conducting pressure fluid from said outlet of said charging pump to said intake of said hydrostatic pump of said first brake circuit and to said intake of said hydrostatic pump of said second brake circuit, during traction slip control; a first check valve disposed between said charging conduit and said return conduit of said first brake circuit and preventing fluid flow from said return conduit of said first brake circuit to said charging conduit; and a second check valve disposed between said charging conduit and said return conduit of said second brake circuit add preventing fluid flow from said return conduit of said second brake circuit to said charging conduit, said first check valve and said second check valve for hydraulically separating said first brake circuit from said second brake circuit.
 2. A brake system in accordance with claim 1 further comprising a drive connected to said charging pump, said hydrostatic pump of said first brake circuit, and said hydrostatic pump of said second brake circuit, for jointly driving said charging pump, said hydrostatic pump of said first brake circuit, and said hydrostatic pump of said second brake circuit.
 3. A brake system in accordance with claim 1, wherein said valve disposed in said connecting conduit is closed in response to said pressure build up of pressure fluid in said master brake cylinder in response to actuation of said brake pedal. 